Externally Gapped Line Arresters (EGLAs) are a type of line arrester used to mitigate the effects of lightning strikes and electrical surges on electrical power line equipment. An EGLA is typically installed in electrical parallel with an insulator that acts to support a power line. With such an EGLA in place, lightning strikes or other types of voltage surges that might cause the insulator to experience flashover are instead diverted to the ground. Damaged equipment and service interruptions are thereby avoided.
While not utilized extensively in the United States, EGLAs have been in production and use in Japan and other foreign countries for several years. A typical EGLA comprises an external gap in series with a series varistor unit (SVU). The SVU, in turn, comprises non-linear metal oxide resistors (MORs) encapsulated in a polymer housing. Because of the non-linear behavior of MORs, the SVU exhibits high resistance at normal operating voltages, but rapidly becomes a low resistance pathway at higher applied voltages such as those produced by lightning strikes. The external gap, because it is arranged in series with the SVU, must spark over before the SVU can begin to conduct electricity.
Unfortunately, it is possible for an SVU in a conventional EGLA to experience a voltage condition during a lightning strike or other surge event sufficient to cause that SVU to fail and not revert back to its original high resistance state when the strike or surge is over. With such a failed SVU, system basic impulse level (BIL) is compromised and the EGLA no longer provides optimal protection for the equipment that it is intended to protect. Nevertheless, because SVUs are normally constructed with polymer housings for purposes of strength and explosion control, there is frequently no obvious outer indication that an SVU has failed. This makes the tracking down and repair of failed EGLAs particularly difficult for the utilities charged with maintaining that equipment.